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From 463c338b09710609e0dc82f67e03c829a7b83788 Mon Sep 17 00:00:00 2001
From: Allan Sandfeld Jensen <allan.jensen@qt.io>
Date: Fri, 14 May 2021 10:43:11 +0200
Subject: [PATCH] Avoid mixing atomic futex changes and QAtomic
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Either the mix of futex and atomic, or the mix of 32-bit futex and
64-bit atomic doesn't work. In any case, the existing code leads to
bad behavior.
* asturm 2021-11-19: Also threw the typo fix from 587e3bb0 into the mix.
Pick-to: 6.1 5.15
Fixes: QTBUG-92188
Change-Id: Icc6ba28d6e2465c373d00e84f4da2b92c037e797
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit 2d9cc639a4a7a5e97979a6034364bd67dfa10c23)
---
src/corelib/thread/qsemaphore.cpp | 46 ++++++++++++-------------------
1 file changed, 17 insertions(+), 29 deletions(-)
diff --git a/src/corelib/thread/qsemaphore.cpp b/src/corelib/thread/qsemaphore.cpp
index d4fb756b94..1d01fc1b28 100644
--- a/src/corelib/thread/qsemaphore.cpp
+++ b/src/corelib/thread/qsemaphore.cpp
@@ -357,47 +357,31 @@ void QSemaphore::release(int n)
quintptr prevValue = u.fetchAndAddRelease(nn);
if (futexNeedsWake(prevValue)) {
#ifdef FUTEX_OP
- if (!futexHasWaiterCount) {
- /*
- On 32-bit systems, all waiters are waiting on the same address,
- so we'll wake them all and ask the kernel to clear the high bit.
-
- atomic {
- int oldval = u;
- u = oldval & ~(1 << 31);
- futexWake(u, INT_MAX);
- if (oldval == 0) // impossible condition
- futexWake(u, INT_MAX);
- }
- */
- quint32 op = FUTEX_OP_ANDN | FUTEX_OP_OPARG_SHIFT;
- quint32 oparg = 31;
- quint32 cmp = FUTEX_OP_CMP_EQ;
- quint32 cmparg = 0;
- futexWakeOp(u, INT_MAX, INT_MAX, u, FUTEX_OP(op, oparg, cmp, cmparg));
- } else {
+ if (futexHasWaiterCount) {
/*
On 64-bit systems, the single-token waiters wait on the low half
and the multi-token waiters wait on the upper half. So we ask
the kernel to wake up n single-token waiters and all multi-token
- waiters (if any), then clear the multi-token wait bit.
+ waiters (if any), and clear the multi-token wait bit.
atomic {
int oldval = *upper;
- *upper = oldval & ~(1 << 31);
+ *upper = oldval | 0;
futexWake(lower, n);
- if (oldval < 0) // sign bit set
+ if (oldval != 0) // always true
futexWake(upper, INT_MAX);
}
*/
- quint32 op = FUTEX_OP_ANDN | FUTEX_OP_OPARG_SHIFT;
- quint32 oparg = 31;
- quint32 cmp = FUTEX_OP_CMP_LT;
+ quint32 op = FUTEX_OP_OR;
+ quint32 oparg = 0;
+ quint32 cmp = FUTEX_OP_CMP_NE;
quint32 cmparg = 0;
+ u.fetchAndAndRelease(futexNeedsWakeAllBit - 1);
futexWakeOp(*futexLow32(&u), n, INT_MAX, *futexHigh32(&u), FUTEX_OP(op, oparg, cmp, cmparg));
+ return;
}
-#else
- // Unset the bit and wake everyone. There are two possibibilies
+#endif
+ // Unset the bit and wake everyone. There are two possibilities
// under which a thread can set the bit between the AND and the
// futexWake:
// 1) it did see the new counter value, but it wasn't enough for
@@ -405,8 +389,12 @@ void QSemaphore::release(int n)
// 2) it did not see the new counter value, in which case its
// futexWait will fail.
u.fetchAndAndRelease(futexNeedsWakeAllBit - 1);
- futexWakeAll(u);
-#endif
+ if (futexHasWaiterCount) {
+ futexWakeAll(*futexLow32(&u));
+ futexWakeAll(*futexHigh32(&u));
+ } else {
+ futexWakeAll(u);
+ }
}
return;
}
--
2.34.0
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